Why is neither /root/.bashrc nor /root/.profile executed after sudo -i?

I have export PATH="$PATH:/opt/local/bin" in /root/.bashrc and /root/.profile (nothing else in /root/.profile) and /bin/bash is the login shell for root in /etc/passwd. Yet invoking sudo -i (which according to man sudo starts a login shell and should run ~/.profile) causes /opt/local/bin to be absent from PATH according to echo $PATH.

EDIT: invoking bash explicitly causes /root/.bashrc to be sourced.

The output of sudo grep 'export PATH=' /root/.bashrc is

export PATH="$PATH:$HOME/.rvm/bin" # Add RVM to PATH for scripting
export PATH="$PATH:/opt/local/bin" # MacPorts

/etc/sudoers (99 % sure I didn't change anything):

Defaults env_reset
Defaults mail_badpass
Defaults secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
root ALL=(ALL:ALL) ALL
%admin ALL=(ALL) ALL
%sudo ALL=(ALL:ALL) ALL

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